How Ultraviolet Radiation Eliminates Bacteria, Viruses, and Protozoa from Water Without Chemicals

Water can appear perfectly clear and yet still carry organisms capable of making us sick. Bacteria, viruses, protozoa, and parasites are among the most common biological contaminants in surface water sources, wells, and even distribution networks. Therefore, in addition to filtering sediments and improving taste, a responsible water treatment system needs a reliable disinfection stage. One of the most effective and cleanest technologies to achieve this is ultraviolet (UV) light.

In this article, we review which biological contaminants may be present in water, how the main disinfection alternatives compare, and why UV light has become the preferred standard in residential, commercial, and industrial applications.

What biological contaminants can be in water?

Water can harbor different types of microorganisms, each with a

different behavior:

• Bacteria: such as E. coli, Legionella, Campylobacter, or Klebsiella, present in surface water, wells, and even poorly maintained distribution networks.
• Viruses: such as rotavirus, norovirus, and hepatitis A, which do not multiply in water but survive in it and are especially resistant to some disinfectants.
• Protozoa and parasites: such as Giardia and Cryptosporidium, which form chlorine-resistant cysts.
• Fungi and algae: which, in addition to posing a health risk, can affect the taste, odor, and color of water.

Exposure to these microorganisms can cause anything from mild gastrointestinal discomfort to serious infections, especially in people with compromised immune systems, children, and the elderly. This is why the disinfection stage is not optional in any water treatment system.

Alternatives to eliminate biological contaminants

Several technologies exist to inactivate or eliminate microorganisms from water, each with advantages and limitations:

• Chemicals (chlorine, peroxide, bromine): effective, but depending on the dose, they can be toxic to people and the environment, and some generate dangerous byproducts.
• Ultraviolet light: does not require chemicals, but requires knowledge of the appropriate energy dose and periodic maintenance of the equipment.
• Filtration (ultrafiltration or osmosis): retains parasites and bacteria by pore size, although simple microfiltration is not always sufficient.
• Ozone: a very powerful oxidant that leaves no harmful residual substances, although its installation is usually more complex.

The chlorine dilemma

For decades, chlorine has been the most widely used disinfectant due to its low cost and its ability to leave residual protection in plumbing. However, when it reacts with organic matter present in water, it can form byproducts such as trihalomethanes (THMs), substances that various studies have linked to an increased long-term risk of certain types of cancer. This is one of the reasons why more and more treatment systems are incorporating UV light as an alternative or complement to chlorine.

Comparing the effectiveness of each method

When comparing disinfection effectiveness, UV light consistently ranks among the best-performing options, alongside ozone and chlorine dioxide, outperforming free chlorine and chloramines against certain resistant microorganisms such as Cryptosporidium.

The effectiveness of any disinfection method is measured in "logarithmic reduction": typical sanitization achieves a 50-99% reduction of microorganisms (2 log), adequate disinfection reaches 99.99% (4 log), and sterilization reaches 99.9999% (6 log). Well-sized UV systems can consistently achieve high levels of disinfection.

What exactly is UV light?

Ultraviolet light spectrum and germicidal effectiveness curve (peak at 254 nm)

Ultraviolet light is electromagnetic radiation with a shorter wavelength than visible light, located between visible light and X-rays in the spectrum. Within the ultraviolet range, the UV-C band (100 to 280 nanometers) has the greatest germicidal capacity, and its optimal effectiveness point is near 254 nm, the wavelength used by most UV disinfection reactors.

How UV light destroys microorganisms

The process is purely physical, without the addition of chemicals. UV-C light penetrates the microorganism's cell wall and alters its DNA or RNA. This does not immediately destroy the cell, but it blocks its ability to replicate, so the microorganism loses the ability to form colonies and cause an infection. Since each microorganism has a different cellular structure, each requires a different level of UV energy to be inactivated; this energy level is called the "UV dose."

The UV dose: the variable that defines everything

The UV dose is calculated as the light intensity multiplied by the exposure time (micro W·sec/cm²). Different reference standards require minimum doses to ensure disinfection: the U.S. public health authority recommends a minimum of 16 mJ/cm², the industry standard is typically 30 mJ/cm², and organizations like NSF/EPA require up to 40 mJ/cm² for certain applications. These values are calculated considering the dose delivered just before the end of the UV lamp's useful life, to ensure that the system remains effective even when the lamp is about to be replaced.

Variables that affect the effectiveness of a UV system

For a UV equipment to disinfect correctly, it is necessary to consider:

• The type and quality of water to be treated (potable, treated wastewater, deionized, etc.).
• The flow rate of water that will pass through the reactor.
• The UV transmittance percentage (%T), which depends on impurities capable of absorbing or reflecting radiation.
• The concentration of iron and manganese, which can reduce light penetration.
• The type and concentration of microorganisms present.
• The desired reduction level according to the final use of the water.

A UV purification equipment should never be operated above the maximum flow indicated by the manufacturer, as this reduces the exposure time and compromises disinfection. Equally important is to respect the lamp replacement intervals recommended by the manufacturer.

Beyond disinfection: other uses of UV in water treatment

UV technology not only inactivates microorganisms; it is also used for:

• Total Organic Carbon (TOC) reduction: using short-wave UV (185 nm) to generate free radicals that oxidize organic matter, key in ultrapure water for pharmaceutical or electronic use.
• Advanced Oxidation Processes (AOP): combining UV with ozone or hydrogen peroxide to degrade substances difficult to treat in wastewater.
• Residual chlorine destruction: a UV reactor can remove up to 15 ppm of free residual chlorine or 5 ppm of chloramines in a single pass, although with a dose 15 to 30 times greater than that used for disinfection.

UV technology

At Hidroteco, we work with high-efficiency reactors designed to offer reliable disinfection with more compact equipment and higher flows. Among their most important features are:

• Automatic identification of the UV lamp, for exact control of its duration and operation.
• Automatic power adjustment, which saves energy, prevents water from heating up, and reduces scaling.
• Contact chambers in 316 stainless steel, which guarantee longer equipment life.
• Screens with exact system information, for easy monitoring of operation.

 

Contact chamber: water flows around the UV lamp, directly exposed to germicidal radiation

This type of technology allows UV disinfection to be integrated as another stage within a complete water treatment system, whether at a residential, commercial, or industrial level, without the need to dose additional chemical products.

Conclusion

UV light disinfection combines proven effectiveness, operational simplicity, and the absence of chemical byproducts, making it one of the best options available today to protect the biological quality of water. Whether you are looking to complement an existing filtration system or design a complete treatment from scratch, understanding how the UV dose works and what variables affect its performance is key to choosing the right equipment.